Groundwater is an important source for drinking water. In the United States in 1990, groundwater provided about 40% percent of water withdrawn for public supply for cities and towns and about 95% percent of water withdrawn by self-supplied systems for domestic use. Nitrates are common chemicals that can pass through soil and potentially contaminate groundwater. Nitrates originate from nitrogen (N2), a plant nutrient supplied by inorganic fertilizer and animal manure. Additionally, airborne nitrogen compounds given off by industry and automobiles can precipitate onto land in rain or as dry particles. Other non-agricultural sources of nitrate include lawn fertilizers, septic systems, and domestic animals.
In agricultural areas, nitrate is the primary form of nitrogen. Nitrate is water-soluble and can easily pass through soil to the groundwater table. The nitrate concentration in groundwater can reach high levels as more nitrogen fertilizer is applied to the land surface annually or more frequently. Nitrate can persist in groundwater for decades. By some estimates, only about one-half of the nitrogen-containing fertilizers applied to agricultural crops is taken up by the crops, and the other half is left in the soil.
Although nitrate generally is not an adult public-health threat, ingestion in drinking water by infants can cause fatally low oxygen levels in the infant's blood, a condition known as methemoglobinemia. Nitrate concentrations in natural ground waters are usually less than 2 mg/L.
In California, nitrate groundwater contamination threatens the drinking water supplies of more than 1.3 million people living in or near agricultural areas. For example, in one sample of 2,500 drinking water wells in agricultural areas, the nitrate concentration exceeded the MCL (Maximum Contaminant Level) in 10% of the wells.
Nitrogen chemistry is complex because nitrogen can have several oxidation states. Moreover, the oxidation states created by bacterial action can be either positive or negative, depending upon whether the bacteria exist in aerobic or anaerobic conditions. The nitrogen cycle illustrates the formation of nitrite ions (NO2-) and nitrate ions (NO3-). Nitrites can be oxidized by Nitrobacter bacteria to form nitrates. Under anoxic conditions, nitrites and nitrates are both reduced by a process call denitrification. During denitrification, some bacteria produce ammonia while others create nitrogen gas. For denitrification to occur, organic material must be present, as it is oxidized for energy while the nitrogen is being reduced.
Like nitrates, water pollution by perchlorate can be a problem. Perchlorate is both a naturally occurring and man-made chemical that is used to produce rocket fuel, fireworks, flares and explosives. Perchlorate can also be present in bleach and in some fertilizers. Some scientific research has indicated that perchlorate contamination of water can disrupt the thyroid's ability to produce hormones needed for normal growth and development. Some suggestions have been made that the safe level of concentration of perchlorate in potable water is less than 10 PPB.